The present invention relates to an internal combustion engine which is charged by two exhaust-gas turbochargers and has exhaust-gas recirculation.
DE 36 33 405 A1 discloses a diesel internal combustion engine, the cylinders of which are arranged in two rows, the exhaust gases from each cylinder row driving an exhaust-gas turbocharger, and the exhaust-gas turbochargers in the known internal combustion engine serving in each case for charging only the cylinders of the associated cylinder row.
DE 44 16 572 C1 discloses an internal combustion engine, the cylinders of which are charged with the aid of two parallel-connected exhaust-gas turbochargers, the exhaust-gas turbochargers being driven by the exhaust gases from the cylinders. Furthermore, the known internal combustion engine has exhaust-gas recirculation, in which part of the exhaust gases is branched off before they enter a turbine of one of the exhaust-gas turbochargers. This part is admixed with the intake air upstream of a compressor of this exhaust-gas turbocharger.
The present invention is concerned with the problem of improving the power behaviour of an internal combustion engine of the type mentioned above.
The exhaust-gas recirculation of the internal combustion engine according to the invention is based on the principle of returning the exhaust gases from the pressure level prevailing upstream of the turbine inlet to the pressure level prevailing downstream of the compressor outlet (if appropriate, downstream of a charge-air cooler). In this case, the separate cylinder rows or cylinder banks are in each case assigned separate exhaust-gas discharge lines which, for recirculating the exhaust gas, open into a common recirculation line. As a result, the exhaust-gas part-quantities from the two cylinder banks which are to be returned are intermixed with one another before they are introduced through the recirculation line into the air supply or admixed with the charge air. This arrangement affords the following advantages.
Only one common air collector, a so-called xe2x80x9cairboxxe2x80x9d, in which only one charge-pressure sensor is arranged, is required for the charge air. Moreover, only one charge regulating circuit is required, which, where appropriate, carries out difference compensation for the two turbochargers. For exhaust-gas recirculation, only one exhaust-gas recirculation regulating circuit is necessary. By appropriate valve means, the circuit regulates or sets the exhaust-gas recirculation rate necessary in each case. Furthermore, in the arrangement according to the invention, a common charge-air cooler can be used. The cooler may expediently be equipped with a common throttle valve, in order, where appropriate, to increase the feed gradient for exhaust-gas recirculation. Moreover, only one exhaust-gas recirculation heat exchanger, which is to be arranged in the common recirculation line, needs to be provided for cooling the recirculated exhaust gases.
Since the exhaust-gas discharge lines assigned in each case to a cylinder bank open into a common recirculation line, a communicating connection is made between the two exhaust-gas discharge lines. The common recirculation line is required, inter alia, in order to achieve pressure equalization between the exhaust-gas turbochargers which do not operate in an entirely identical way. An essential feature of the internal combustion engine according to the invention is stop means by which the communicating connection made between the two exhaust-gas discharge lines can be shut off.
This measure is based on the knowledge that, when the internal combustion engine is in a relatively low rotational speed range, pressure pulsations occur in the cylinder banks. Because of the communicating connection between the exhaust-gas discharge lines, these pulsations generate or induce mass oscillations. On account of these mass oscillations, some of the pulsation energy to the overall exhaust-gas energy is lost in the form of wall heat losses and frictional losses in the interconnected exhaust-gas discharge lines. The result of this is that only a relatively low charge pressure can build up in the relatively low rotational speed range mentioned, so that a comparatively low torque is available. This leads to the performance of the internal combustion engine likewise being relatively low.
Pulse charging essentially takes place in the low rotational speed range mentioned. At higher rotational speeds, however, only ram charging takes place. In ram charging, the mass oscillations mentioned, along with the accompanying consequences, do not occur. During the acceleration of the internal combustion engine, the transition from pulse charging to pure ram charging has a significantly noticeable effect, a power jump of this kind being undesirable.
With the aid of the stop means proposed according to the invention, this communicating connection between the exhaust-gas discharge lines can be broken or shut off, with the result that the pressure pulsations cannot generate any mass oscillations, so that the losses caused thereby do not occur. A considerable power rise or torque increase is consequently obtained for the internal combustion engine according to the invention in the low rotational speed range mentioned.
By designing the internal combustion engine in the way according to the invention, when the internal combustion engine is at relatively low critical rotational speeds, the exhaust-gas discharge lines can be separated from one another, and the exhaust-gas turbochargers can then be charged completely independently of one another, thus resulting in a maximum power rise for the internal combustion engine. On the other hand, in non-critical rotational speed ranges, the advantages of the common simple regulating circuits for exhaust-gas recirculation can be utilized.
Furthermore, should the valve means serving as an exhaust-gas recirculation valve be arranged upstream of an exhaust-gas heat exchanger arranged, if appropriate, in the recirculation line, it is possible to ensure that, when the stop means are opened, only hot exhaust gas flows through the valve means, thus preventing the situation in which condensate, possibly occurring in the exhaust-gas heat exchanger, leads to sooting-up problems in and sticking of the valve means.
The performance of an internal combustion engine according to the invention can be increased in a particular way, in that, when the internal combustion engine is in a relatively low rotational speed range, the communicating connection between the exhaust-gas discharge lines is shut off by the stop means. Exhaust-gas recirculation can then also be shut off. With the aid of these measures, the abovementioned power rise of the internal combustion engine is obtained in the low rotational speed range in which the exhaust-gas discharge lines are shut off. In this context, the fact that, where appropriate, no exhaust-gas recirculation serving for exhaust-gas emission control takes place during this time can be ignored. This is because the increased performance or increased available engine torque makes it possible to leave this engine operating phase quickly by virtue of the increased acceleration capacity. Immediately after leaving the critical rotational speed range, the stop means can connect the exhaust-gas discharge lines to one another again, and exhaust-gas recirculation, cut off if appropriate, can be reactivated, so that the internal combustion engine then works in engine operating ranges in which optimum exhaust-gas emission control can take place.
Other important features and advantages of the invention may be understood from the claims, from the drawings and from the accompanying figure description with reference to the drawings.
The features mentioned above and those still to be explained below can be used not only in the combination specified in each case, but also in other combinations or alone, without departing from the scope of the present invention.
Preferred exemplary embodiments of the invention are illustrated in the drawings and are explained in more detail in the following description.